Thermoplastic polymers, such as polystyrene, styrene-maleic anhydride copolymers, and impact-modified versions of these, have many uses. Often, the usefulness of these materials is limited by their inherent combustibility. One conventional approach to improving flame retardancy of thermoplastics is to add high levels of halogen and/or phosphorus-containing salts, compounds, or polymers. These flame-retardant additives can adversely impact certain physical properties of the thermoplastic polymers, such as heat resistance, impact strength, and tensile strength. Also, the additives can lose their effectiveness by leaching out of the polymer. Halogenated flame retardants are particularly troublesome because they can produce toxic and corrosive gases in a fire.
A second approach to making flame-retardant thermoplastics is to chemically incorporate the halogen or phosphorus into the polymer chains. This can be accomplished after polymer synthesis, for example, by reacting the polymer with a phosphorus reagent (see, e.g., U.S. Pat. No. 3,993,635). Preferably, chemical incorporation is effected by preparing the polymer in the presence of a copolymerizable phosphorus- or halogen-containing monomer. Although chemical incorporation is usually preferred over the additive approach, it has several drawbacks. Special, expensive monomers may be needed; many of these monomers lack the favorable reactivity ratios required for copolymerization to give polymer products that have a high enough level of phosphorus or halogen to be sufficiently flame retardant.
Kraft et al. (U.S. Pat. Nos. 3,725,509 and 4,014,836) teach copolymers useful as flame-retardant additives for thermoplastic polymers. The copolymers are made from a halogen-containing unsaturated monomer (vinyl halide, halogenated styrene, etc.) and a bis(hydrocarbyl) vinyl phosphonate. Optionally, the copolymer includes one or more other copolymerizable monomers such as .alpha.-olefins, acrylates, dicarboxylic acids or anhydrides, vinyl aromatic monomers, and the like. The copolymer additives can be used as flame retardants for a wide variety of thermoplastics.
Younes (U.S. Pat. Nos. 4,444,969 and 4,571,418) teaches copolymers of vinyl aromatic monomers, bis(hydrocarbyl) vinyl phosphonates, and imide derivatives of unsaturated anhydrides (maleimide, N-phenylmaleimide, etc.).
Brunner et al. (U.S. Pat. No. 4,035,571) teach flame-retardant copolymers of a bis(hydrocarbyl) vinyl phosphonate, an unsaturated monomer, and acrylic or methacrylic acid.
Unfortunately, the poor mechanical properties of vinyl phosphonate polymers, especially those that incorporate a large amount of phosphorus, limits their usefulness per se as flame-retardant polymers.
Flame-retardant thermoplastic polymers are needed. Preferably, the polymers have both high phosphorus content and good mechanical properties. Also desirable are flame-retardant compositions that incorporate the flame retardant into the polymer chains so that additives are not needed.